1. Field of the Invention
The present invention relates to a reversible thermosensitive recording material capable of recording and erasing images repeatedly by utilizing the property of reversibly changing the transparency from a transparent state to an opaque state, and vice versa, depending upon the temperature thereof.
2. Discussion of Background
Recently attention has been paid to a reversible thermosensitive recording material capable of temporarily recording images thereon and erasing the same therefrom when such images become unnecessary. As representative examples of this kind of reversible thermosensitive recording material, there are conventionally known reversible thermosensitive recording materials in which an organic low-molecular-weight material such as a higher fatty acid is dispersed in a matrix resin such as vinyl chloride--vinyl acetate copolymer, as disclosed in Japanese Laid-Open Patent Applications 54-119377 and 55-154198.
The conventional resin employed in the above reversible thermosensitive recording material has poor heat resistance since the content of the vinyl chloride repeat unit is about 80 to 91 wt. %, and the average polymerization degree thereof is about 400.
In the case where only heat energy is applied to the reversible thermosensitive recording material by using a heat-application roller or a heat-pen, with application of slight pressure thereto, in order to perform recording and erasing operations, the durability of the recording material is not a problem even when the image formation and erasure are repeated. However, when both heat and pressure are applied to the recording material at the same time, for instance, by using a thermal head, the durability of the recording material becomes a problem during the repeated image formation and erasing operations. This is because the matrix resin has poor heat resistance. More specifically, the matrix resin around the particles of the organic low-molecular-weight material in the recording layer is deformed and the particle size of the organic low-molecular-weight material dispersed in the matrix resin is increased during the course of the repeated recording and erasing operations. As a result, the light scattering effect of the recording layer is decreased. As a matter of course, the degree of whiteness of a white opaque portion in the recording layer is also significantly decreased. In the end, the image contrast is disadvantageously lowered.